1. Field of the Invention
The present invention relates to a method and apparatus for receiving discrete parts travelling at a speed and applying the parts to a web travelling at a different speed. The invention more particularly concerns a method and apparatus for receiving discrete parts of a continuously moving web of material travelling at a certain speed and applying the parts to a second continuously moving web travelling at a different speed.
2. Description of the Related Art
Articles, such as disposable diapers, generally have been manufactured by a process where discrete parts or components of different materials, such as leg elastic, waist elastic, tapes and other fasteners such as hook and loop materials or snaps, have been applied to a continuously moving product web. Often, the speed at which the parts are fed into the process is not the same as the speed of the product web itself. Thus, the speed of the parts must be changed to match the speed of the product web to properly apply the parts without adversely affecting the process or the finished product.
Several different conventional methods for changing the speed of a part or component of material such that it can be applied to a continuously moving web have been known to those skilled in the art.
For example, one method has been known as the slip gap or slip cut method. A web of material, which is travelling at a slower speed than the moving web, is fed into a knife and anvil roll having a surface speed equal to the speed of the moving web. As the material is cut into discrete parts, vacuum in the anvil roll is activated to draw the parts of material to the surface of the anvil roll. The anvil roll then carries the parts to the moving web where the vacuum is released and the parts are applied to the moving web while both the parts and the moving web are travelling at the same speed.
Another method has utilized festoons to reduce the speed of the moving web to match the speed of the discrete parts of material to be applied to the web. The moving web is temporarily slowed down to the speed of the parts with the excess portion of the moving web gathering in festoons. The parts of material are then applied to the moving web while both the parts and the web are travelling at the same speed. The festoons are then released allowing the moving web to return to its original speed.
Another method has utilized a slider-crank mechanism to accomplish the speed change. The slider-crank mechanism utilizes concentrically mounted arms or linkages to receive the discrete parts of material, increase the speed of the parts to match the speed of the moving web and apply the parts to the moving web. The slider-crank mechanism is a special case of a four bar linkage system.
Finally, another such method to change the speed of a discrete part before it is applied to a moving web has utilized a cam actuated crank-follower mechanism. The cam actuated crank-follower mechanism comprises levers that are mounted on a rotatable driving plate. Each lever has a pivot point and includes a cam follower on one end and a drag link on the other end. An applicator device is connected to the other end of the drag link. The cam follower remains in contact with a fixed cam that is mounted concentric with the driving plate's center of rotation. As the driving plate rotates, the levers pivot as their cam followers follow the cam shape. As the levers pivot, the applicator devices are caused to speed up or slow down. Thus, the mechanism can be designed to receive discrete parts of material, change the speed of the parts and apply the parts to a moving web. An example of this method is described in U.S. Pat. No. 4,610,751 issued Sep. 9, 1986, to Eschler.
Conventional methods, such as those described above, have exhibited several drawbacks. First, as the discrete parts of material are transferred, they are often subjected to a tugging action because the surface speed of the transfer means used to transfer the parts is greater than the speed of the parts. The tugging action may result in an elongation or tear of the parts. Second, several of the conventional methods provide substantial speed variations but do not provide any periods where the speed remains constant for a fixed duration. Thus, the discrete parts may be adversely affected because the surface speed of the transfer means used to transfer the parts is continuously changing during the receiving and application process. Finally, several of the conventional methods can be very expensive and time consuming to change as the size and speed of the discrete parts and the speed of the moving web change to coincide with various finished product sizes. Consequently, an inexpensive and adaptable method for receiving discrete parts travelling at a speed and applying the parts to a web travelling at a different speed is desirable.
Moreover, it is desirable that the receiving and applying of the parts occurs while the respective surface speeds are maintained substantially constant for a fixed duration. For example, it is desirable to apply the parts to the substrate web while the parts and substrate web are travelling at substantially the same surface speed. A constant speed dwell allows precise control of the length and placement of the part on the substrate web especially if the part is fragile and/or elastic.